Nucleic acids (DNA=deoxyribonucleic acid, RNA=ribonucleic acid) are frequently used as starting materials for various analyses and assays in medical and pharmaceutical research, clinical diagnosis and genetic fingerprinting which typically require high quantity nucleic acids input. As a matter of routine, adequate quantities of nucleic acids may readily be obtained by means of in-vitro amplification techniques, e.g., based on the polymerase chain reaction (PCR).
Amplification of nucleic acids based on PCR has been extensively described in patent literature, for instance, in U.S. Pat. Nos. 4,683,303, 4,683,195, 4,800,159 and 4,965,188. Basically, the PCR includes a multiply repeated sequence of steps for the amplification of nucleic acids, wherein in each sequence                nucleic acids are melted (denaturated) to obtain denatured polynucleotide strands,        primers are annealed to the denaturated polynucleotide strands, and        the primers are extended to synthesize new polynucleotide strands along the denaturated strands to thereby obtain new copies of double-stranded nucleic acids.        
Due to the fact that reaction rates in the PCR-reactions vary with temperature, the samples are cycled through predefined temperature profiles in which specific temperatures are kept constant for selected time intervals. The temperature of the samples typically is raised to around 90° C. for melting the nucleic acids and lowered to approximately 40° C. to 70° C. for primer annealing and primer extension along the denaturated polynucleotide strands.
In daily routine, automated apparatus (thermal cyclers) are being used for cycling the reaction mixtures through the temperature excursions which typically include a temperature-controlled block used for heating or cooling the nucleic acids containing samples. As, for instance, is described in US-patent application 2005/0145273 A1, temperature-control of the block typically involves the use of thermoelectric heating and cooling devices utilizing the Peltier effect (“Peltier devices”). Connected to a DC power source, each of the Peltier devices functions as a heat pump which can produce or adsorb heat to thereby heat or cool the samples depending upon the direction of electric current applied. Accordingly, the temperature of the samples can be changed according to a predefined cycling protocol as specified by the user applying varying electric currents to the Peltier devices.
Conventional Peltier devices usually can be cycled several ten thousand times until failure is likely to occur. As detailed in above US-patent application, Peltier devices may experience fatigue of solder joints electrically connecting individual pellets each Peltier device typically is provided with, resulting in an increase of the electric resistance of the Peltier devices which in turn aggravates fatigue to thereby cause rapid failure.
In modern thermal cyclers, failure of a Peltier device is an error which causes a current run to be stopped normally requiring the samples running for amplification to be discarded. However, since the nucleic acids containing samples may be unique in a sense that they can hardly or even not be re-obtained such as in certain forensic applications, accidental stops due to failure of Peltier devices must be avoided. Hence, Peltier devices have to be replaced in good time before failure is likely to occur.
Conventionally, Peltier devices are replaced after a preset number of thermal cycles performed. As a considerable variability in non-failure cycles of Peltier devices is experienced, a convenient trade-off between expected life-time and risk of failure must be found which, on the one hand, increases costs as Peltier devices may be replaced too early and, on the other hand, decreases liability of the thermal cycler since failure of at least some Peltier devices may not be prevented.
In order to solve that problem, the above-cited US patent application discloses a method in which upon each initial turn-on of the thermal cycler an AC resistance of the Peltier devices is measured to detect their likeliness to fail. More specifically, based on storing an AC resistance time history of each of the Peltier devices, a currently measured resistance value of an individual Peltier device is compared with a previously measured resistance value of the same Peltier device, and, in case AC resistance of the Peltier device increases by 5% with respect to the previous record, it is assumed that the Peltier device will soon fail and is marked to be replaced.
As a matter of fact, AC resistance values of the Peltier devices markedly depend on external influences such as ambient temperature requiring such influences to be compensated using complex correction algorithms. Hence, such method involves rather complicated calculations and is thus difficult to perform and, due to the fact that the liability of the results depends on the correction algorithms chosen, may not be significant either.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide an improved method for monitoring Peltier devices in thermal cyclers which is easy to perform, reliable in use and helps save costs in identifying Peltier devices which are likely to fail soon so that these Peltier devices can be selectively replaced in a timely manner.